The present invention is related to transducers, and more particularly, to the type of transduces which converts electrical input signals to a pressure output, i.e., a double-acting electropneumatic positioner.
Electropneumatic positioners are devices which set up a connection between electrical or electronic devices and pneumatic actuators. An input signal representing a command variable is a small load-independent current in the mA range (e.g., 4-20 mA), whereas the regulated quantity (i.e., the output signal) is a controlled air pressure which is used for the actuation of a pneumatic diaphragm or piston actuators.
Electropneumatic positioners commonly operate according to the force-balancing principle. An electrical signal, a load independent current, provided by a controller (or steering device) to the electropneumatic positioner is coverted into a pneumatic signal proportional to the electric signal via a moving coil and a nozzle/baffle plate system, or via a piezo beam which is deflectable by the electrical control signal. This pneumatic signal, after being amplified, serves to control the pneumatic actuator.
With respect to present double-acting electropneumatic positioners, it is common to provide a nozzle/baffle plate system in which two nozzles are acting together with one baffle plate. With an increasing pressure at one nozzle a corresponding decreasing pressure follows at the other nozzle with this pressure being applied to the actuator after being amplified. Such a pneumatic positioner is known from FIG. 1 of German utility Model 75 38 863. With the use of a piezo beam a second inversely controlled piezo beam must be provided.
Besides the increased costs, the known device shows the disadvantage that one of both systems always has an increased air consumption and that due to tolerances of the components used, the dynamic behavior may be different so that the pressure within oppositely arranged chambers of the actuator, in certain cases, can show a behavior which is not strictly counter current.
Thus, there is a need to provide a double-acting electropneumatic positioner having low cost and having essentially ideal, consistent performance characteristics.